1. Field of the Invention
The present invention relates to a CMOS image sensor and a method for fabricating the same, and more particularly, to a CMOS image sensor and a method for fabricating the same in which the boundary between an active region and a field region is not damaged by ion implantation.
2. Discussion of the Related Art
Generally, an image sensor means a semiconductor device converting optical images into electrical signals. The image sensor is divided into a charge coupled device (CCD) and a complementary MOS (CMOS) image sensor. The CCD transfers and stores charge carrier to and in a capacitor in a state that respective MOS capacitors lie adjacent to each other. The CMOS image sensor employs a switching mode that provides MOS transistors by the number of pixels using CMOS technology based on peripheral circuits such as a control circuit and signal processing circuit and that detects output signals of the MOS transistors.
The CCD has several drawbacks. That is, the CCD requires much power consumption and its driving mode is complicated. Also, since a lot of mask process steps are required, a signal processing circuit cannot be realized within a CCD chip.
To solve such drawbacks, studies of a CMOS image sensor based on sub-micron CMOS technology have been progressed recently. In the CMOS image sensor, images are realized by forming a photodiode and a MOS transistor within a unit pixel and detecting signals in a switching mode. In this case, less power consumption is required because the CMOS technology is used. Also, since twenty masks are required, the process steps are simpler than those of the CCD, which require thirty to forty masks. Thus, a signal processing circuit can be integrated within a single chip. This enables a small sized product and various applications of the product.
A related art CMOS image sensor will now be described with reference to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 are a circuit diagram and a layout illustrating a unit pixel structure of a related art CMOS image sensor. Although three or more transistors constituting a CMOS image sensor may be used, for convenience, a CMOS image sensor based on three transistors will be described.
As shown in FIG. 1 and FIG. 2, a unit pixel 100 of the CMOS image sensor includes a photodiode 110 and three NMOS transistors. The photodiode 110 serves as a sensor. Of the three transistors, a reset transistor Rx 120 transfers optical charges generated in the photodiode 110 and discharges the charges to detect signals. Another driver transistor Dx 130 serves as a source follower. Other select transistor Sx 140 is for switching and addressing.
Meanwhile, in the image sensor of the unit pixel, the photodiode 110 serves as a source of the reset transistor Rx 120 to facilitate charge transfer. To this end, the process steps of fabricating an image sensor of a unit pixel include the step of lightly or heavily implanting impurity ions into a region including some of the photodiode as shown in FIG. 2.
The process steps of fabricating an image sensor of a unit pixel corresponding to the section taken along line A-A′ of FIG. 2 will be described with reference to FIG. 3A to FIG. 3C. For reference, a solid line of FIG. 2 denotes an active region 160.
As shown in FIG. 3A, a gate insulating film 122 and a gate electrode 123 are sequentially formed on a P type semiconductor substrate 101 in which a device isolation film 121 is formed by a shallow trench isolation (STI) process. In this case, although not shown, a P type epitaxial layer may previously be formed in the P type semiconductor substrate 101. Subsequently, a photoresist film is deposited on the entire surface of the semiconductor substrate 101. A photoresist pattern is then formed by a photolithography process to define a photodiode region. At this time, the gate electrode is not exposed by the photoresist pattern.
In this state, lightly doped impurity ions, for example, N type impurity ions are implanted into the entire surface of the semiconductor substrate to form a lightly doped impurity ion region in the semiconductor substrate 101 at a predetermined depth.
Subsequently, as shown in FIG. 3B, another photoresist pattern 125 is formed and a lightly doped impurity ion region for an LDD structure is formed in a drain region of the gate electrode using the photoresist pattern 125 as an ion implantation mask. At this time, the lightly doped impurity ion region is not exposed by the photoresist pattern 125.
Afterwards, as shown in FIG. 3C, a spacer 126 is formed at sidewalls of the gate electrode 123, and a P type impurity ion region P0 is formed on the N type impurity ion region n−. Thus, the process steps of forming a photodiode are completed. In a state that the photodiode is completed, heavily doped impurity ions are selectively implanted into the drain region of the gate electrode 123 to form a heavily doped impurity ion region n+. Finally, the process steps corresponding to the section taken along line A-A′ of FIG. 2 are completed.
In the method for fabricating the related art CMOS image sensor, lightly doped impurity ions are implanted into the active region and the device isolation film to form a photodiode. At this time, a defect occurs in a corresponding substrate due to ions implanted into the boundary between the device isolation film and the active region.
This defect due to ion implantation causes charge or hole carrier and provides a charge-hole recombination area, thereby increasing leakage current of the photodiode. That is, dark current occurs, in which electrons are transferred from the photodiode to the floating diffusion region in a state that there exists no light. The dark current is caused by either various defects, which are generated in the vicinity of the surface of silicon, the boundary between the device isolation film and P0, the boundary between the device isolation film and n−, the boundary between P0 and n−, P type region, and n− type region, or dangling bond. The dark current also deteriorates low illumination characteristics of the CMOS image sensor.
In the U.S. Pat. No. 6,462,365, a device isolation film and a transfer gate are formed in a portion corresponding to a photodiode region to reduce dark current generated by damage of a photodiode. Besides, while other various methods for minimizing dark current have been suggested, there is no effective method for solving a defect caused at the boundary between the device isolation film and the active region due to ion implantation.